The present invention relates to a field of optical record reproducing, particularly to an optical record reproducing device having a separated optical system.
FIG. 22 shows one of embodiments of a conventional optical record reproducing device. As shown in the drawing, in the optical record reproducing device, although a movable optical system 102, capable of moving a radial direction C of an optical disc, 109 such as a photomagnetic disc employs an objective lens 108 and a reflective mirror 107, a light source (not shown) is excluded. The light source and a reflective mirror 101 are located at a fixed position with respect to the optical disc 109. Accordingly, a laser beam irradiated from the light source is reflected at the reflective mirror 101 and introduced to the reflective mirror 107 and the objective lens 108 in the optical system 102. In this embodiment, prisms are used as the reflective mirrors 101, 107 and the beam is totally reflected by the prisms.
In the embodiment, an angle of the reflective mirror 101 is accurately displaced by a tracking actuator (not shown). That is, the reflective mirror 101 is held with a holder 101a and the holder 101a can be inclined or pivoted with respect to a rotational direction E of which a central axis is in a direction A parallel to the irradiated light beam from the light source by a supporting mechanism. An accurate angle displacement mechanism for accurately displacing an angle of the holder 101a and the reflective mirror 101 a predetermined degree along the rotational direction E is utilized as a tracking actuator. Generally, a reflective mirror for displacing angle accurately is called as a galvanomirror. In the present invention, such a reflective mirror is called as the galvanomirror.
One embodiment of a supporting mechanism of the galvanomirror is shown in FIGS. 23(a) and 23(b). As shown in the drawings, a metal plate 113 is mounted on a substrate 112 and a rubber supporting member 114 is attached at an upper end of the metal plate 113. The galvanomirror 115 is a plate-shaped reflective mirror. At both sides of the supporting member 114, a coil 116 is located, respectively and a magnet 117 is located outside of each coil 116 with a predetermined interval. The accurate angle displacement mechanism is constituted by these coils 116 and magnets 117. Accordingly, when predetermined electric magnetic force is applied to a space between the coil 116 and the magnet 117 by conducting the coil 116, the supporting member 114 and the galvanomirror 115 are pivotally inclined on a portion engaging with the metal plate 113. Accordingly, a laser beam irradiated from the galvanomirror 115 is moved by driving the accurate angle displacement mechanism so as to displace the angle of the galvanomirror accurately.
Accordingly, in the optical record reproducing device as shown in FIG. 22, a laser beam 103 irradiated from a light source such as a semiconductor laser and so on is irradiated on the galvanomirror from a direction of an arrow A and the laser 101 is reflected toward a direction of an arrow B as a reflected laser beam 104. The laser beam 104 is reflected by the reflective mirror 107 in the movable optical system 102 and returned toward a direction of an arror D as a laser beam 105 and the laser beam 105 is condensed by the objective lens 108 and a light spot 111 is formed on an optical disc 109 so as to be utilized for optical record reproducing. A location of the light spot 111 is slightly displaced in a range of less than several hundreds .mu.m by accuratly displacing an angle of the galvanomirror 101 and a reflective direction of the laser beam 104 so as to operate a tracking control for controlling the location of the light spot corresponding to a track 110.
As described above, a separated optical system in which a movable optical system 102, including the objective lens 108, and a fixed optical system including the light source, are separated from each other has a tracking actuator in the fixed optical system so that the weight of the movable optical system is relative low and the moving speed is attendantly increased.
On the other hand, the movable optical system 102 is linearly moved along a radius direction C of the optical disc 109 by a seeking actuator. A rough position displacement mechanism for linearly displacing the movable optical system along the radius direction within a range of several tens mm is used as the seeking actuator so as to search tracks from the innermost track to the outermost track of the optical disc 109. For example, a voice coil motor (hereinafter it is called as "VCM") is used as the rough position displacement mechanism.
In the optical record reproducing device as shown in FIG. 22, although the accurate angle displacement mechanism, that is, the tracking actuator is employed as a tracking control for controlling a light spot 11 corresponding to a track 110, if the light spot 111 is disposed outwardly or inwardly from the track 110 a relatively large amount, there is a limit in regard to the ability of the light spot to be moved by the accurate angle displacement mechanism alone.
Therefore, it has been developed a tracking control system by combining the accurate angle displacement mechanism and a rough position displacement mechanism so as to increase a follow-up control of the light spot 111 in tracking control. For example, a tracking control system comprises the accurate angle displacement mechanism and the rough position displacement mechanism which are connected in parallel. A respective detecting signal from an error detecting means (hereinafter, it is called as "a tracking error signal") is divided by a signal distributor and distributed to the accurate angle displacement mechanism and the rough position displacement mechanism. And, a tracking error signal is directly applied to the accurate angle displacement mechanism and a signal, which is a filter processed tracking error signal by an equivalent filter of the accurate angle displacement mechanism, is applied to the rough position displacement mechanism, so that a controlable region of a displacement is equivalent of a region of the control system divided by dependent connection.
In the tracking control, the accurate angle displacement mechanism and the rough location displacement mechanism are dependently connected via a signal transmitting means, the follow-up control is increased and there is a merit of reducing a sliding amount from an optical axis. Hereinafter, such a control system is referred to as a two-stage tracking control.
In the conventional optical record reproducing device as shown in FIG. 22, the direction C of moving the movable optical system 102 and the reflective direction B at a mechanically neutral point in the rotational direction of the galvanomirror 101 have to be parallel each other.
However, in prior art, there is no means for directly detecting an angle displacement of the galvanomirror 101. Therefore, the device is constructed by parts which are produced accurately with high attention so as to prevent the direction C of moving the movable optical system 102 from not being parallel to the reflective direction B of the galvanomirror 101 at the neutral point due to dispersion of used parts of the galvanomirror 101 and others and control error and to locate the direction C strictly parallel to the reflective direction B.
However, there is a limit for producing the parts and controlling accurately, so that it is difficult to locate the moving direction C strictly parallel to the reflective direction B and it generally results in angular error 0. By ocurring the angular error 0, the laser beam is outwardly shifted from a center line of the objective lens and a on-track condition is started while the laser beam is inclined or slided with respect to the optical disc 109.
Therefore, even if the tracking error signal is zero, the center line of the objective lens is in an off-set condition. Even if a tracking control for the optical disc 109 is operated, a light spot condensed by the movable optical system is not located at a center line of a track. In addition, while information is read, crosstalk occurs. While information is written, the writing is not done at a correct position. After producing the device, the angular error 0 becomes large, since a respective part is influenced by enviromental change and spending time, so that the problem becomes more serious.
In the supporting mechanism of the conventional galvanomirror as shown in FIGS. 23(a) and 23(b), rubber material is used as the supporting member 114. The supporting member has a low hold-rigidity for directions except a direction for rotating the galvanomirror 101 and the supporting member is easily influenced by dispersion and resonance frequency of the supporting member is low so that a control performance of the galvanomirror is reduced.
In the conventional tracking control system, a feed back control is operated by a tracking error signal only and it is necessary to use a filter which is equivalent to the filter of the signal distributor and the accurate angular displacement device, so that structure of a circuit becomes complete. Further, a characteristic of a whole control is determined by natural resonance frequency of the accurate angular displacement mechanism and the rough position displacement mechanism. Therefore, it is a subject to control the characteristic of the whole circuit without adjusting the natural resonance frequency of the accurate angular displacement mechanism and the rough position displacement mechanism.
In addition, in the two-stage tracking control, it is impossible for a method for two-stage tracking control by only the error detecting system to independently control the angle of the galvanomirror as the accurate angular displacement mechanism in the feed back control time and the two-stage tracking control system sometimes becomes unstable.
A subject of the present invention is developed to resolve the above described prior art. A purpose of the present invention is to provide an optical record reproducing device for reducing an influence of disturbance and controling resonance frequency by a tracking control for adjusting angular error 0 by detecting a rotational displacement of the galvanomirror and increasing the hold rigidity of the galvanomirror along all directions except the rotational direction by providing the supporting member which is made of synthetic resin and has a thin wall hinge portion. Further, another purpose of the present invention is to adjust a characteristic of the whole control system to a preferable characteristic by controlling the tracking error signal in feed back control and further by detecting the rotational displacement of the galvanomirror and inputing its data to the rough position displacement mechanism and the accurate angular displacement mechanism.